The present invention relates to a method for the preparation of a diorganopolysiloxane blocked at each molecular chain end with a silanolic hydroxy group or, more particularly, to a method by which a diorganopolysiloxane end-blocked with silanolic hydroxy groups and having a desired viscosity can be easily and reproducibly prepared leaving little hydrolyzable groups even from inexpensive starting materials.
As is well known, diorganopolysiloxanes blocked at each molecular chain end with a silanolic hydroxy group are Widely used, for example, as a base ingredient of room temperature-curable silicone rubber compositions. In the prior art, diorganopolysiloxanes blocked at each molecular chain end with a silanolic hydroxy group are prepared by a method described below. Namely, the starting material in the conventional method is a cyclic diorganosiloxane oligomer, such as octamethyl cyclotetrasiloxane, and the diorganosiloxane oligomer is first subjected to a pretreatment for dehydration or drying as completely as possible by stripping under reduced pressure or by blowing dry nitrogen gas into the liquid oligomer at an elevated temperature prior to the ring-opening polymerization reaction to equilibrium in the presence of an alkali catalyst. In the next place, the thus dehydrated diorganosiloxane oligomer is admixed with an exactly controlled amount of water and an alkali catalyst and the mixture is heated at a specified temperature for a specified length of time to effect the ring-opening polymerization reaction until equilibrium is established followed by neutralization of the alkali catalyst and stripping of the low molecular-weight constituents and other volatile matters therein to give a diorganopolysiloxane blocked at each molecular chain end with a silanolic hydroxy group and having a desired viscosity or degree of polymerization. It is important in this process that an exactly controlled amount of water is contained in the oligomer before the polymerization reaction because the viscosity or degree of polymerization of the diorganopolysiloxane obtained by the reaction is determined by the content of water or the molar ratio of the water to the diorganosiloxane units in the oligomer in the reaction mixture.
It is of course possible in this prior art method that an undehydrated or incompletely dehydrated cyclic diorganosiloxane oligomer or a linear-chain diorganosiloxane oligomer having silanolic hydroxy groups at the molecular chain ends is used as the starting material. It is essential in this case, however, that the content of the residual water or the amount of the silanolic hydroxy groups relative to the amount of the diorganosiloxane units is exactly controlled in order that the resulting diorganopolysiloxane product may have a desired viscosity.
Another problem in the above described prior art method is that the type of the terminal groups at the molecular chain ends of the diorganopolysiloxane product is influenced when the starting diorganosiloxane oligomer contains hydrolyzable functional groups besides the silanol groups.
As is understood from the above description, the prior art method for the preparation of a silanol-terminated diorganopolysiloxane has disadvantages because the process as a whole is complicated or lengthy and requires utmost carefulness for exactly controlling the contents of water or silanol groups in the starting materials or elimination of functional groups other than silanol groups in the starting materials sometimes resulting in a poorly reproducible quality of the products and an increase in the production costs.